The function of human organs such as the lung, kidney and vascular system are critically dependent on cells forming tubes of the correct diameter and length. However, the mechanisms controlling tube size are poorly understood. This is reflected in a lack of effective treatments for many human diseases in which tube-size control is defective, such as polycystic kidney disease, and our inability to control tube size to treat diseases that are not directly due to tube-size defects. For example, drugs that block vascular tube-size growth could be used as anti-angiogenic drugs to block solid tumor growth. The proposed research would define three aspects of the multiple mechanisms that control tube- size using the Drosophila tracheal as a model system. Drosophila trachea are a ramifying network of epithelial tubes that function as a combined pulmonary/vascular system and provides a powerful molecular/genetic system for investigating the basic mechanisms of tube-size control using in vivo approaches and techniques that are difficult or infeasible with vertebrate models. Our data show that members of the conserved Src kinase family control the orientation of cell growth during development of the Drosophila trachea and the embryonic mouse aorta, and that the Drosophila formin dDAAM works with Src42 to orient tracheal cell growth. Strikingly, Src42 and dDAAM form a complex that defines a new polarity pathway that is distinct from known apical/basal polarity and planar cell polarity pathways. The experiments proposed for the first aim would determine whether Src42 and dDAAM act to send or to receive signals that orient growth, whether dDAAM acts as a regulator or an effector of Src, and identify effectors of the Src42 and dDAAM orientation function. In the second aim, we will test the hypothesis that increases in cell volume regulate the extent of tracheal cell apical membrane and identify which genes and pathways mediate the ability of Src42 and dDAAM to control the tracheal apical surface area. For the third aim, our preliminary data show that the transcription factor Yorkie, which acts in conserved cell growth and proliferation pathways, controls tube size through the cell death (apoptosis) gene named Drosophila inhibitor of apoptosis (DIAP), despite the fact that there is no apoptosis in the tracheal system. In this aim, we will test that hypothesis that Yorkie is the effector of one or more of the five known pathways that controls tracheal tube size, as well as test the hypothesis that DIAP acts through a subset of canonical apoptosisfector proteins to regulate tube size. Together, the results of the proposed experiments will provide critical insights into the molecular mechanisms that regulate tube-size control, which should ultimately lead to the ability to therapeutically manipulate tube size.